The present invention relates to rotary fluid pressure devices, and more particularly, to such devices which include gerotor displacement mechanisms.
Although the present invention may be used advantageously with gerotor devices which are to be used as fluid pumps, the invention is especially advantageous when utilized as part of a gerotor motor, and particularly those of the low speed, high torque type, and will be described in connection therewith. In addition, the invention is especially advantageous when utilized as part of a gerotor device intended to operate at relatively higher pressures and torques.
Furthermore, although the present invention may be used advantageously with gerotor motors having various types of valving, it is especially advantageous when utilized in a high pressure motor of the "valve-in-star" (VIS) type, and will be described in connection therewith. An example of a VIS motor is illustrated and described in U.S. Pat. No. 4,741,681, assigned to the assignee of the present invention and incorporated herein by reference. In a VIS motor, commutating valving action is accomplished at an interface between an orbiting and rotating gerotor star, and an adjacent, stationary valve plate, which is typically either part of the motor housing (or end cap), or comprises a separate member, but is held rotationally stationary relative to the motor housing. An example of a VIS motor in which the stationary valve member is a member separate from the motor housing is illustrated and described in U.S. Pat. No. 4,976,594, also assigned to the assignee of the present invention and incorporated herein by reference.
Increasingly, low speed, high torque gerotor motors of the kind to which the invention relates, are expected to be able to perform well even in the presence of relatively high back pressures, i.e., a pressure substantially above reservoir pressure at the return (outlet) port of the motor. As is well known to those skilled in the art, high back pressures are common in the case of closed circuit vehicle propel systems in which the system charge pressure is being increased to improve the performance of the servo system which controls the displacement of the hydrostatic, propel pump. As is also well known, the system charge pressure inherently determines the back pressure at the motor, because charge pressure ("make-up" fluid) is communicated to the low pressure side of the system, which is the outlet side of the propel motor.
An inherent characteristic of VIS type motors is that the back pressure exerts a separating force on the gerotor star, tending to separate the star (which is the orbiting and rotating valve member) from the adjacent valving surface on the stationary valve member. As is well known to those skilled in the gerotor motor art, such separation of adjacent valving surfaces will substantially reduce the volumetric efficiency of the motor, the volumetric efficiency being the ratio of the actual output of the motor to the theoretical motor output which would have been, if there had been no leakage within the motor. It has been determined that for certain VIS motor configurations, the star separation issue is not as much of a problem at elevated system pressures, because system pressure is used to bias the gerotor star toward the adjacent surface of the stationary valve member. Instead, the problem may be most noticeable at relatively lower system pressures, when there is less resulting biasing force on the star. It is believed that the problem may be exacerbated by the relatively high bolt torque which is used in view of the fact that the motor is intended for relatively higher pressure applications. The high bolt torque can have the effect of distorting the prior art balancing plate, thus opening up leakage clearances between the gerotor and the balancing plate, and reducing volumetric efficiency. Of greater concern is the fact that the bolt torque results in an unpredictable preload on the balancing plate, in view of variations in factors such as thread finish, etc., whereas what is really desired is a known, predictable preload.
Accordingly, it is an object of the present invention to provide an improved low speed, high torque gerotor motor, and especially a motor of the VIS type, which is able to perform satisfactorily, even in the presence of a relatively higher back pressure, with less of a decrease in volumetric efficiency.
It is another object of the present invention to provide a VIS type gerotor motor having an improved balancing plate and seal arrangement which makes it possible to reduce the gerotor side clearance, for further increased volumetric efficiency, while at the same time, effectively increasing the side clearance tolerance band, thus reducing the manufacturing cost of the gerotor.
Is has been observed that the effort to reduce gerotor side clearance, and increase volumetric efficiency can have one undesirable effect. Increasing the loading on a balancing plate disposed adjacent the forward surface (i.e., the end opposite the stationary valve plate) of the star can result in galling between the end surface of the star tooth and the adjacent surface of the balancing plate, especially at a location of high relative velocity between the adjacent surfaces. As is well known to those skilled in the gerotor motor art, any galling between relatively moving parts is likely to lead fairly quickly to total inoperability of the motor.
Accordingly, it is another object of the present invention to provide an improved gerotor motor which has an increased ability to prevent galling between the end surfaces of the gerotor star and the adjacent surface of the balancing plate.
It is a more specific object of the present invention to provide an improved gerotor motor which achieves the above-stated object by directing pressurized fluid to the area subject to galling, thus cooling and lubricating the area of potential galling.